Technical Field of the Invention
Aspects of the present disclosure relate in general to optical spectrometers, and in particular to Fourier Transform Infrared (FTIR) micro spectrometers.
Description of Related Art
Absorption spectrometers measure how well a sample absorbs light at each wavelength. Most absorption spectrometers utilize a “dispersive spectroscopy” technique, but others utilize a “Fourier transform spectroscopy” technique. The main difference between a FTIR spectrometer and other dispersive-type spectrometers (or spectrometers based on tunable wide-free-spectral-range high-finesse Fabry-Perot filters) is that an FTIR spectrometer measures all the wavelengths coincidentally, while other types of spectrometers measure one wavelength a time. As a result, FTIR spectrometers have higher measuring speeds and larger signal to noise ratios than dispersive spectrometers.
FTIR spectrometers are typically based on Michelson interferometers, in which collimated light from a broadband source is split into two beams, which are then reflected off of respective mirrors (one of which is moving) and caused to interfere, allowing the temporal coherence of the light to be measured at each different Optical Path Difference (OPD) offered by the moving mirror. The resulting signal, called the interferogram, is measured by a detector at many discrete positions of the moving mirror. The measured spectrum is retrieved using a Fourier transform carried out by a processor.
The interferogram of a single wavelength coherent source is periodic and varies with the OPD by a cosine function. Ideally, measuring any part of the interferogram would result in the same spectrum. Broadband sources, however, have most of the interferogram power concentrated around the zero OPD. Therefore, the moving mirror travel range should cover this portion of the interferogram. This is usually achieved by letting the respective distances between the beam splitter and each of the mirrors be equal (or close to it) and moving the mirror such that the distance between the beam splitter and the moving mirror assumes both negative and positive values with respect to the OPD position resulting in the detection of a double-sided interferogram. The maximum travel range scanned by the moving mirror (i.e. actuator travel range) governs the resolving power of an FT spectrometer. The larger the travel range, the better the resolution such that the resolution is inversely proportional to the travel range.
Many versions of the FTIR spectrometer based on Michelson interferometry have been developed based on the motion of an in-plane mirror or out-of-plane mirror with respect to the substrate. FTIR spectrometers based on Fabry-Perot (FP) interferometers or Mach-Zehnder interferometers, instead of Michelson interferometers, have also been developed.
Micro-optical bench technology provides an excellent platform for highly-integrated, self-aligned and electromechanically scanned interferometers. It enables the design, validation, and fabrication of monolithic optical systems on a single silicon chip. The principal technology is based on Deep Reactive Ion Etching (DRIE) of silicon-on-insulator (SOI). The height of the micromirrors in the deeply-etched micro-optical benches is usually limited such that beyond this limit, the verticality of the etched surface deteriorates with a tilted profile and significantly rough surface. The optical throughput of the devices is directly related to the size and solid angle by which the device is accepting the optical energy from the source. At the same time, the signal-to-noise ratio of the sensor, such as an FTIR spectrometer device with micro-optical components, is directly related to its optical throughput of the components, if the rest of the system is optimized. This is especially true due to the low brightness of the wideband sources used in spectroscopy. Therefore, what is needed is a spectrometer device with an increased optical throughput.